Mechanistic Variants

Other mechanistic variants occur with the additions of F2 and CI2. Although anti 1,2-addition products dominate the reactions with CI2 and Br2, one also often finds allylic halo-genation with CI2. Radical pathways occur in these reactions, and so the addition is not typically as clean as with Br2. In contrast to both CI2 and Br2, the addition of F2 occurs via a syn addition mechanism. The prevailing data support the formation of a p-fluorocarbocation that rapidly combines with the fluoride counterion before dissociation of the ion pair (Eq. 10.22). 

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Generally, when two different aryl units are involved in a coupling reaction, the oxidation potentials of the two units will be different. In another mechanistic variant, the more easily oxidized phenol 14 can react with the other unperturbed aromatic unit 15 to furnish a biaryl radical 16, which then oxidizes further, deprotonates, and finally tautomerizes to give the product biaryl (Scheme 4). 

Although this is not conclusive evidence for the ylide-free mechanistic variant, it is at least strongly in favor of it. It is worth mentioning that in the same paper the momentary ee world record for this type of reaction was reported (Scheme 100). 

The rearrangement of 1-methoxy-substituted bicyclo[2.2.2]octenones (66a-d) follows a mechanistic variant of the ODPM reaction, which includes two consecutive single-step phototransformations, i.e. the ODPM path to (67a-d), again triplet sensitized, and conversion to the final 1,4-diketones (68a-d) cf. Scheme 13).5-1U05-108 jjjg second step involves cleavage of the transient cyclopropane in (67a-d) accompanied by loss of the methoxy methyl, most likely via methyl radicals, and transfer of a hydrogen atom from the solvent. 

In a final example, use has been made of a mechanistic variant of the ODPM reaction the conversion of a 1-methoxy-substituted bicyclo[2.2.2]octenone (92) to the bicyclo[3.3.0]octadione (94 Scheme 23). The highly substituted p,y-enone (92) is accessible via hydroxymethylation of (66c), a compound which... 

A major remaining issue in ACPC deaminase catalysis is the mode of actual cyclopropane fragmentation. At least three mechanistic variants can be pictured (A) p-proton abstraction to initiate cyclopropane fragmentation (B) nucleophilic addition to... 

Simulations that have been carried out for a vast number of different rate parameters and mechanistic variants show that only the slow second-order coupling steps between dimers and the subsequent slow elimination of protons can explain the trace-crossing (Fig. 2). In any case, the so-called nucleation loop in cyclic voltammetry is not the result of a nucleation step, but of slow, homogeneous follow-up reactions of soluble oligomers in the diffusion layer. 

Cherkasov, R. A., Polezhaeva, N. A., Galkin, V. I. Arbuzov reaction in the series of halogenocyclenes new synthetical and mechanistical variants. Phosphorus, Sulfur Silicon Relat. Bern. 1999, 144-146, 333-336. 

Phosphorous ylides are almost the only representative of a mechanistic variant. Rather than being displaced in an internal Sn2 reaction, the phosphorus bonds to oxygen and then undergoes an elimination (path 4e) as the very stable phosphine oxide. 

Ivanov et al. studied the MPV reaction between ethanol and acetone over various metal oxides having different acid-base properties [13]. Reaction was found to occur over both Lewis acid (AI2O3-CI) and base (MgO, Zr02) catalysts. Based on FT-IR and specific poisoning experiments, two mechanistic variants were proposed differing in the mode of formation of... 

Other mechanistic variants decouple the transfers of one or both electrons from the transfer of the proton so that, for example, the reaction of Eq. (4.2a) might be accomplished by an initial proton transfer followed by a sequence of two one-electron transfers or the transfer of an electron succeeded by the transfer of a hydrogen atom. These processes or combinations of steps within them may be called formal hydride-transfer reactions. 

Previously, tin-ketyl radicals have been added to alkenes only in an intramolecular fashion. [9] In recent publications, however, pinacols and amino alcohols have been prepared by cyclisation of dicarbonyl compounds [10] or keto-oximes [11] with tributyltin hydride. Cyclisation of 1,5-ketoaldehydes 1 and 1,5-dialdehydes with tributyltin hydride yields cw-diols 2 with excellent stereoselectivities, whereas the keto-oxime 4 with four benzyloxy-substituents affords a 58 42 cis trans) mixture. The tran -product was transformed in two more steps to the potent glycosidase inhibitor 1-deoxynojirimycin (6). [lib] The reversibility of both the addition of the tributyltin-radical to the carbonyl group and the intramolecular radical C-C bond formation is believed to be responsible for the high selectivity in the formation of 2. In the cyclisation of 1,5-pentanedial the unhydrolyzed coupling product 3 could be isolated, therefore providing evidence for a new mechanistic variant of the pinacol reaction, in which only 1.2 equivalent of the reducing agent are necessary. 

Although the above remains the mechanistic paradigm for enzymatic phosphoryl transfer, other mechanistic variants have been uncovered since the original breakthroughs these, however, are outside the scope of our discussion. 

As illustrated in Scheme 6.13, a range of other R-X substrates can be coupled with heterocycle formation, as can carbon monoxide insertion to form keto-substituted products (Scheme 6.13b) [22]. These are typically postulated to proceed in an analogous fashion to the reactions above. In addition to these, several mechanistic variants have also been developed. For example, Zhang has reported that allyl-substituted furans can be synthesized in a multicomponent reaction of ketone 11, allylic halides and various nucleophiles (Scheme 6.16) [23]. This reaction utilizes the... 

An interesting mechanistic variant of styrene-amine addition upon direct irradiation is presented by the reaction of the o-hydroxystyrene 28 with MeNHj to yield the adduct 28a in 62% yield (Scheme 17). The initial step in the reaction mechanism is deprotonation of the acidic singlet state of 28 by the amine to yield a phenolate-ammonium ion pair. Reprotonation of the phenolate ion yields a zwitterion, which is trapped by the nucleophilic amine. Thus, the amine serves both as a base and as a nucleophile in this reaction. 

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